In the application of air conditioning and heat pump systems significant improvements in operating efficiencies are attained, especially in mild weather operation when the capacity of the equipment is reduced while utilizing all of the heat transfer surface of the system. It is well known to provide two or more compressors connected in parallel to a single refrigeration system, various ones of which may be cycled on and off to vary the capacity of the system.
When a plurality of compressors is connected in parallel in a single refrigeration system, as when there is a common suction line, oil circulating through the system with refrigerant may not be returned to the several compressors in the proper proportions. It may be necessary, therefore, to provide some arrangement for equalizing or distributing the oil among the several compressors. Generally, it is necessary to provide an oil equalizer line between crankcases of multiple compressors and connected at a predetermined height to the compressor that is always running when the system is in operation such that at least a minimum oil level at that height is maintained in that compressor when both compressors are running and that a suitable level of oil is maintained in the not-running compressor when only one compressor is running. Another problem arises when a cycled-off compressor is turned on to start running while another compressor running in the system has created a system pressure against which the starting compressor must start.
Another prior art system of providing capacity modulation is to employ one two-speed motor-compressor in a single shell. This arrangement solves the problems of oil distribution encountered in the multiple compressor systems and that of starting a second compressor against the pressure created by the running compressor. However, these systems do not provide optimum efficiency at all speeds and further require controls that are capable of switching motor speed.
It is an object of the present invention to provide a capacity modulating refrigerant system that employs a single shell compressor that has optimum efficiency at all running capacities.